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Water boils when the vapor pressure equals atmospheric pressure at the water's surface. The vapor pressure increases with temperature. So you might reason that with a lid on, the pressure at the surface of the water would increase a bit, which would mean that you need higher temperature to get the vapor pressure up a bit higher. Getting the temp up a bit higher would take a bit longer, so using this line of reasoning you would expect the water to take longer to boil.

But that's wrong.

With no lid, you lose quite a bit of energy as the hot vapor evaporates from the surface and leaves the pot. You lose significantly less when you put a top on the pot. You don't stop the loss completely becasue the typical lid is a loose fit and not well-insulated, but a lid on the pot helps quite a bit anyway. With a lid, more of the the heat from your stove goes into raising the temperature (and thus the vapor pressure) of the water, and LESS is lost out the top, so it reaches the boiling point sooner.

Secondly, having a typical loose fitting lid on a pot doesn't really increase the pressure at the surface of the water to any significant degree. A pressure cooker or car radiator is another story because they're well sealed.

So yes, covering the pot will boil your water faster.

In a related fashion, if you have to leave your coffee for a few minute, put something over the top of the cup. Most of the heat loss is from the surface of the coffee (same as the pot of water). So if you retard the evaporative loss by putting something over the cup, it'll stay warmer quite a bit longer. A porcelain tile works quite well!

Now, sometimes you want your water to boil at a higher temperature to cook your food faster. Which is what you use a pressure cooker for. By sealing the vessel, the pressure increases and thus the boiling point goes up.

Your car radiator works the same way. Of course you should have antifreeze in there, which has a higher boiling point to start with. But the pressurization allows the system to run even hotter before you get a boil-over.

Another way to cook your pasta at a higher temperature is to salt the water a bit. When you add an impurity (the salt in this case) to water, the boiling point elevates and the freezing point depresses. Salted water will take a bit longer to boil than pure water, but it will be a few degrees hotter than pure water.

Water boils when the vapor pressure equals atmospheric pressure at the water's surface. The vapor pressure increases with temperature. So you might reason that with a lid on, the pressure at the surface of the water would increase a bit, which would mean that you need higher temperature to get the vapor pressure up a bit higher. Getting the temp up a bit higher would take a bit longer, so using this line of reasoning you would expect the water to take longer to boil.

But that's wrong.

With no lid, you lose quite a bit of energy as the hot vapor evaporates from the surface and leaves the pot. You lose significantly less when you put a top on the pot. You don't stop the loss completely becasue the typical lid is a loose fit and not well-insulated, but a lid on the pot helps quite a bit anyway. With a lid, more of the the heat from your stove goes into raising the temperature (and thus the vapor pressure) of the water, and LESS is lost out the top, so it reaches the boiling point sooner.

Secondly, having a typical loose fitting lid on a pot doesn't really increase the pressure at the surface of the water to any significant degree. A pressure cooker or car radiator is another story because they're well sealed.

So yes, covering the pot will boil your water faster.

In a related fashion, if you have to leave your coffee for a few minute, put something over the top of the cup. Most of the heat loss is from the surface of the coffee (same as the pot of water). So if you retard the evaporative loss by putting something over the cup, it'll stay warmer quite a bit longer. A porcelain tile works quite well!

Now, sometimes you want your water to boil at a higher temperature to cook your food faster. Which is what you use a pressure cooker for. By sealing the vessel, the pressure increases and thus the boiling point goes up.

Your car radiator works the same way. Of course you should have antifreeze in there, which has a higher boiling point to start with. But the pressurization allows the system to run even hotter before you get a boil-over.

Another way to cook your pasta at a higher temperature is to salt the water a bit. When you add an impurity (the salt in this case) to water, the boiling point elevates and the freezing point depresses. Salted water will take a bit longer to boil than pure water, but it will be a few degrees hotter than pure water.

Sort of, but not exactly. You said that the lid acts as insulation, but pan lids are not really insulators. The major heat loss mechanism is water vapor escape.

If we assume the lid works by "insulation", your comment that at an air temp of 212F a lid wouldn't matter would be correct. But since the heat loss is due to escape of vaporized water, the lid will help even at high temperatures. Now, if the air was at 212F and 100% relative humidity, then a lid wouldn't matter.

As for the microwave, whether the lid helps or hurts depends on the material of the lid (usually not metal in a microwave). If you use a lid that reflects or absorbs S-band microwaves, the lid is likely to slow down the boiling. If you have a lid that is effectively transparent to the radiation, it'll speed things up for the same reason it helps on a stove.

Small points, I know, bit in my world "what's happening" is just as important as "what happens" if you get my drift. Sorry for all the words, but no worries, I don't charge by the word.

Water boils when the vapor pressure equals atmospheric pressure at the water's surface. The vapor pressure increases with temperature. So you might reason that with a lid on, the pressure at the surface of the water would increase a bit, which would mean that you need higher temperature to get the vapor pressure up a bit higher. Getting the temp up a bit higher would take a bit longer, so using this line of reasoning you would expect the water to take longer to boil.

But that's wrong.

With no lid, you lose quite a bit of energy as the hot vapor evaporates from the surface and leaves the pot. You lose significantly less when you put a top on the pot. You don't stop the loss completely becasue the typical lid is a loose fit and not well-insulated, but a lid on the pot helps quite a bit anyway. With a lid, more of the the heat from your stove goes into raising the temperature (and thus the vapor pressure) of the water, and LESS is lost out the top, so it reaches the boiling point sooner.

Secondly, having a typical loose fitting lid on a pot doesn't really increase the pressure at the surface of the water to any significant degree. A pressure cooker or car radiator is another story because they're well sealed.

So yes, covering the pot will boil your water faster.

In a related fashion, if you have to leave your coffee for a few minute, put something over the top of the cup. Most of the heat loss is from the surface of the coffee (same as the pot of water). So if you retard the evaporative loss by putting something over the cup, it'll stay warmer quite a bit longer. A porcelain tile works quite well!

Now, sometimes you want your water to boil at a higher temperature to cook your food faster. Which is what you use a pressure cooker for. By sealing the vessel, the pressure increases and thus the boiling point goes up.

Your car radiator works the same way. Of course you should have antifreeze in there, which has a higher boiling point to start with. But the pressurization allows the system to run even hotter before you get a boil-over.

Another way to cook your pasta at a higher temperature is to salt the water a bit. When you add an impurity (the salt in this case) to water, the boiling point elevates and the freezing point depresses. Salted water will take a bit longer to boil than pure water, but it will be a few degrees hotter than pure water.

yeah yeah, I thought about this after i typed it, the boiling point IS higher once steam has elevated the atmospheric pressure inside the lidded container, albiet maybe only slightly depending on the weight/seal of the lid.

The lid allows for more heat retention, reaching the boiling point faster.

yeah yeah, I thought about this after i typed it, the boiling point IS higher once steam has elevated the atmospheric pressure inside the lidded container, albiet maybe only slightly depending on the weight/seal of the lid.

The lid allows for more heat retention, reaching the boiling point faster.

Comment

Think of it like this: at the surface of any substance there will be higher and lower energy molecules. Higher energy molecules will occasionally escape the solid or liquid parent material and escape into the surrounding environment. As these higher energy molecules leave, they decrease the overall energy of the parent material because they have taken energy with them. If the surrounding environment is saturated with other high energy molecules that have left, the chances of them bouncing off the other free molecules and being absorbed back into the parent material increases. When these molecules are reabsorbed back into the parent material the energy is returned and the overall energy of the parent material is increased again.

So leaving the lid on the pot of water creates a saturation area where these molecules can collect, collide, and return to the water for reabsorbtion thereby retaining the energy in the water and reaching the boiling point faster. Pressure difference for a lid is negligible. I looked at my saturated steam tables and it would take you 2.49 psi to raise the boiling point 8 degrees F. I think at best your going to get maybe 0.1psi out of a lid without it being clamped down so the difference is really going to be small. Empirical evidence shows smaller than the effect of heat retention due to saturation. Sure it depends on the size of the pot, the area, the BTU input, etc, etc...

Water for sure boils a lot faster under vacuum. That's how we made water on ships. We had to keep it at least 168 F to kill any biologicals but when we went to the boiler make up tanks we could crank it down to about 152 F and still make very low salinity water. Of course we used steam powered air ejecetors and brought it down to as good a vacuum as we could. Probably 28.5" Hg or so.

Conversely it has a higher boiling point under pressure. Some of the old ships I worked had B&W sectional header jobs that ran 400 psi and 400 deg F and some others 600 and 600. The "D Tube" types with a high steam and low water drum separate were up to 1200 psi and 900 or so F. Biggest steam plant I personally worked on was a 32,000 HP on a T-10 some years ago.

Anyways steam and water is cool stuff. I went to Cal Maritime and sailed deep sea as a 3rd and 2nd Engineer before I became a land lubber and contractor so I could be around my kids. Sometimes I miss it but I think I would miss my kids more.

Comment

The boiling point of water increases with increased pressure. A lid will increase the pressure proportional to the weight of the lid. So a pot could be heated to say 215, and not actively boiling. Take the lid off and it goes crazy.

The boiling point of water increases with increased pressure. A lid will increase the pressure proportional to the weight of the lid. So a pot could be heated to say 215, and not actively boiling. Take the lid off and it goes crazy.

Yes. Once it's boiling. However, it will take longer to reach that point due to the escaping energy if the lid is left off.